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Ultrasonic degradation of 2:4:6 trichlorophenol in presence of TiO2 catalyst
Ultrasonics Sonochemistry 8 (2001) 227±231
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Ultrasonic degradation of 2:4:6 trichlorophenol in presence of TiO2 catalyst Aniruddha B. Pandit *, Parag R. Gogate, Sukti Mujumdar Chemical Engineering Section, University Department of Chemical Technology (UDCT), Matunga Road, Matunga, Mumbai 400 019, India
Abstract The degradation of trichlorophenol has been studied at dierent intensities of irradiation using ultrasonic horn by changing the power input to the system. Eect of presence of catalyst TiO2 and concentration of catalyst on the degradation rates has also been investigated. The rates of degradation are found to be higher at higher intensities in the absence of catalyst but reverse trend is observed in the presence of catalyst. Adsorption and desorption characteristics of trichlorophenol on TiO2 catalyst have been examined. The catalyst enhances the rates of degradation but it also adsorbs some amount of TCP during the degradation process protecting it from ultrasonic degradation. Thus it is essential to consider the adsorption and desorption kinetics to get proper estimates of the degradation rates when the rates of degradation of TCP are calculated by analyzing the residual concentration of the compound in the liquid. The degradation of the pollutant seems to take place in the liquid only and that too only after desorption of the same from the solid particles. Solid particles seem to play a physical role in the overall degradation scheme, providing additional surface cavitation. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Waste water treatment; Ultrasonic irradiation; Adsorption and desorption characteristics; 2:4:6 trichlorophenol
1. Introduction The development of sonochemistry revealed a new and promising application for hazardous chemical destruction from industrial wastewater. Several complex compounds that have been successfully degraded, though on a laboratory scale at present, include 2-, 3and 4-chlorophenol [1], phenol [2], dichloromethane and o-dichlorobenzene [3], p-nitrophenol [4], etc. Sonication of aqueous solution provokes formation, growth and collapse of cavities resulting into local high pressures and temperature known as hot spots, which are responsible for the chemical degradation. Among the several modes of reactivity, pyrolytic decomposition theory is most widely accepted. According to this theory, during the formation of bubbles, resulting from expansion of cavities present in solution under sound ®eld, vapour from liquid medium or dissolved organic compound penetrate into these bubbles. These bubbles pulsate in the oscillating pressure ®eld till they reach a critical resonant size. At this stage the bubble can no
*
Corresponding author. Fax: +91-22-414-5614. E-mail address: [email protected] (A.B. Pandit).
longer eciently absorb energy from the sound ®eld to sustain itself. The bubbles then implode releazing large amount of energy, which is sucient to break strong chemical bonds in the compounds present in the bubbles as vapour. In aqueous solution, highly reactive OH & H radicals which are generated due to cavitation bombard the organic compound either inside or outer side of the bubbles according to the condition of the reaction and the half-life of the radicals resulting into oxidation reaction. On collapsing, the radicals present in the cavities are thrown into the bulk solution and are likely to be in contact with some dissolved organic molecules in the bulk resulting into oxidation reaction. Thus pyrolysis depends on the localization of solute (i.e. in the bulk, inside the cavities or in the interfacial layer) and therefore on its physico-chemical properties. Henglein and Kermann [5] reported that the hydrophobicity of solution is more responsible for the penetration of compound into the bubble as compared to liquid vapour pressure. Thus, hydrophilic organic compounds may get attacked by OH radical in the bulk solution or in the interfacial ®lm. In other words, hydrophobic and volatile compounds are destroyed very easily inside the bubble whereas non-volatile and hydrophilic compounds
1350-4177/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 5 0 - 4 1 7 7 ( 0 1 ) 0 0 0 8 1 - 5
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A.B. Pandit et al. / Ultrasonics Sonochemistry 8 (2001) 227±231
grade TiO2 catalyst to study its eect. The amount of the catalyst used was varied in the range 0.01 g TiO2 /100 ml to 0.05 g TiO2 /100 ml of TCP solution. The adsorption isotherm of TiO2 catalyst was also studied by dispersing it in aqueous solution of TCP using Ace horn at 30% amplitude which gives maximum dispersion followed by the analysis of free TCP using the spectrophotometer. Desorption kinetics of TiO2 was also studied at 50°C which was the temperature reached due to heating of solution under uncontrolled sonication so that the equilibrium conditions could be studied. The time required to reach 50°C temperature from initial temperature of 30°C was 5 min for 30% amplitude of ultrasonic horn.
are more dicult to oxidize by ultrasound. Thus there is a need to improve the reactivity of these compounds which can be done partly by optimization of the geometry of sonoreactors and operational variables viz., frequency and intensity of ultrasound [6]. Accelerating the rate of the degradation of aqueous organic compounds by adding catalysts is another possibility to increase the eciency of degradation process. In the present work degradation of 2:4:6 trichlorophenol (TCP) has been studied applying ultrasound of dierent intensity and in the presence of a catalyst (TiO2 ) since TCP is well known as a hazardous toxic compound obtained from the euent of various chemical industries. The catalyst TiO2 has been purposefully chosen in the study to facilitate the use of UV irradiation in future as rate of degradation with combination of ultrasound and ultraviolet are higher than due to ultrasound or ultraviolet irradiation alone [7].
3. Results and discussion 3.1. Eect of presence of catalyst Table 1 shows the eect of presence of catalyst on the TCP destruction process. It is found that the degradation of TCP is higher under the in¯uence of the ultrasound in the presence of the catalyst TiO2 compared to that when catalyst was absent. This is due to the fact that the presence of heterogeneous catalyst increases the rate of formation of cavities by providing additional nuclei and also the number of cavities and radicals are increased. The cavitational threshold are also likely to be lowered in the presence of trapped vapour gas nuclei in the crevices of the particular catalyst [8]. Another possible explanation for the observed eect lies in the phenomena of interfacial cavitation at the catalyst surface. Due to the high pressure and temperature pulses generating as a result of cavitation phenomena, fragmentation of catalyst occurs which again provides more number of nuclei thereby increasing the number of cavities and consequently the degradation of TCP present in the solution. It can be also seen from Table 1, that the extent of degradation is also dependent on the concentration of the catalyst present in the reaction mixture. This can be attributed to the fact that the number of nuclei increased as a result of heterogeneous surface and the extent of interfacial cavitation also increases with the quantity of the catalyst. Moreover the rate of degradation of the
2. Experimental The solution (100 ml of 100 ppm concentration of TCP) was sonicated by ultrasonic horn (Ace Glass Inc. USA, operating frequency 22:7 kHz and power rating 600 W) in a pulse mode (5 s on and 1 s o) at room temperature. A schematic representation of the experimental setup has been depicted in Fig. 1. The effect of intensity was studied using dierent amplitude (10%, 20% and 30%) of ultrasonic horn. By changing the amplitude of irradiation, the power input to the system is changed keeping the area of irradiation constant. The residual concentration of TCP was determined using UV/VIS spectrophotometer at a wavelength of 293 nm at intervals of 30 min during a total irradiation time of 2 h. Experiments were repeated in presence of anatase
Fig. 1. Schematic representation of the experimental set-up.
Table 1
Eect of presence of catalyst on the destruction of TCP Time (min)
50 150
Percentage decrease in concentration of TCP Without catalyst
In presence of the catalyst Concentration 0:01 g/100 ml
Concentration 0:05 g/100 ml
11.07 32.12
24.23 66.6
40.37 82.12
Experimental conditions: amplitude 10%, power input to system 60 W.
A.B. Pandit et al. / Ultrasonics Sonochemistry 8 (2001) 227±231
organic compound is proportional to the amount of OH and H radicals produced and present in the reaction mixture and the presence of TiO2 catalyst enhances the dissociation reaction of H2 O thereby increasing the number of free radicals generated. 3.2. Eect of intensity of irradiation Intensity of irradiation can be de®ned as the ratio of the power input to the system to the area of transmitting surface. Intensity of irradiation can be changed in two ways; ®rst by changing the power input or second by changing the irradiating surface area. An increase in the irradiating surface area results into more uniform dissipation of energy and that too over a wider area resulting into more eective of cavitation [6]. Thus the cavitational yield is inversely proportional to the intensity of irradiation in this case. Gogate and Pandit [6] have shown using numerical simulations of the cavity dynamics that, the collapse pressure generated due to the collapse of individual cavity decreases with an increase in the intensity of irradiation and recommended the use of lower intensity i.e. just above a certain optimum required for the inception of cavitation de®ned as threshold intensity. In the present case, the intensity of irradiation has been increased by increasing the power input to the system. The eect of intensity of irradiation is distinctly dierent for the two cases viz., one in absence of catalyst and other in presence of catalyst, which is discussed in details in the following sections. 3.2.1. In absence of catalyst TiO2 In the degradation of TCP solution, it was observed that in the absence of catalyst, the extent of degradation increases with an increase in the power of the sonic horn. This is due to the increased cavitational activity at higher levels of power input. Due to an increase in the power input to the solution, higher number of cavities are generated resulting into higher magnitudes of total
229
pressure pulses (pressure pulse due to collapse of single cavity number of cavities) though the pressure pulse due to collapse of single cavity decreases [6]. Thus the overall cavitational yield increases as is observed in the present work. However it should be noted that the increase in the number of collapsing (eective) cavities with an increase in power input is only upto a particular intensity after which the coalescence and buoyancy renders the additional number ineective. Gogate et al. [9] have clearly shown this fact with experimentation on the decomposition of aqueous solution of potassium iodide. 3.2.2. In the presence of catalyst TiO2 The eect of intensity on the destruction rates in presence of catalyst was studied at two catalyst concentrations viz., 0.01 g/100 ml of solution and 0.05 g/100 ml. The trends in variation of degradation rates for the concentration of 0.01 g/100 ml of solution (Table 2, Case II) were similar to those observed in the absence of catalyst. However, during sonication of TCP with 0.05% TiO2 , the degradation of TCP was less at higher power input, which was quite contrary to the expectation (Table 2, Case III). The observed variation can be explained on the basis of the adsorption and desorption characteristics of TCP on TiO2 catalyst. At higher power input, though the degradation rate in the solution increases, the rate of release of already adsorbed TCP also increases thereby giving lower overall degradation rate based on the free concentration of the TCP in the liquid. The detailed explanation of this fact has been made in the later section on the basis of adsorption and desorption characteristics of TCP on TiO2 . 3.3. Adsorption and desorption characteristics The extent of adsorption of TCP by TiO2 was found to increase with time of contact of TiO2 and TCP solution (Table 3, Part I). During irradiation, the particles of TiO2 experience fragmentation due to shock waves
Table 2 Eect of intensity of irradiation on the degradation rates Time (min)
Percentage decrease in concentration of TCP 30% amp Power 180 W
20% amp Power 120 W
10% amp Power 60 W
42.12 74.05
24.49 48.96
11.07 32.12
Case II: In the presence of catalyst (0.01 g/100 ml of solution) 50 83.7 150 95.2
75.9 89.2
24.23 66.60
Case III: In the presence of catalyst (0.05 g/100 ml of solution) 50 19.32 150 76.85
34.23 80.37
40.37 82.12
Case I: Without presence of catalyst 50 150
230
A.B. Pandit et al. / Ultrasonics Sonochemistry 8 (2001) 227±231
Table 3 Adsorption and desorption characteristics TCP on the catalyst TiO2 Time (h)
Concentration of free TCP (ppm)
ppm adsorbed/gm of catalyst
Part I: Adsorption isotherm of TCP on TiO2 (concentration of 0.05 g/100 ml of solution) 0 78.67 1 26.65 2 22 3 19.92 Time (h)
Concentration of TCP (ppm) Temperature 65°C
Temperature 50°C
Part II: Study of desorption of TCP at elevated temperatures 3h 18.75 3 h 30 min 39.19 4h 42.35 Time for irradiation (min)
± 1040.4 1133.4 1175.5
18.31 24.46 25.68
Concentration of TCP (ppm) With 30% amp power 180 W
With 10% amp power 60 W
Part III: Eect of ultrasound on equilibrated TCP (containing TiO2 ) solution 0 21.12 25 44.02 50 36.30 75 28.5
generated by imploding cavities in the bulk of the solution. The shock waves damage, fragment and deform the solid surface leading to enhancement of surface area and improvement of mass transport due to turbulent mixing and acoustic streaming from the bulk to the catalyst surface. Desorption characteristics were studied at 65°C and 50°C which are the temperatures reached due to heating caused during uncontrolled irradiation with 30% and 10% amplitude respectively in 15 min irradiation time (Table 3, Part II). It can be easily seen from the table, that there is an increase in the concentration of free TCP in the solution after irradiation of the equilibrated sample which con®rms the fact that desorption is also taking place. When the equilibrated sample of TCP and TiO2 was irradiated with ultrasound at dierent amplitudes (Table 3, Part III), it was observed that upto certain irradiation time, the concentration of TCP increases in the solution because of desorption of the TCP from TiO2 present in the solution under the in¯uence of ultrasound. Afterwards, the concentration of TCP increases gradually with time for lower irradiation power (10% amplitude) while, for higher power input (30% amplitude), it decreases with time as expected and observed in the absence of catalyst. This can be explained considering the rates of simultaneous degradation and desorption during sonication. The rate of degradation is less as compared to the rate of desorption from the catalyst in the case of lower irradiation power which ensures that there is more supply of TCP by desorption as compared to removal by degradation thereby increasing the overall
19.19 41.30 52.44 53.49
Table 4 Analysis of individual contribution of degradation and desorption Time (min)
25 50 75
With 30% amp Power 180 W
With 10% amp Power 60 W
% Degradation
% Desorption
% Degradation
% Desorption
34.14 42.12 50.11
12.97 12.76 2.6
5.80 11.07 16.33
35.18 45.07 45.91
concentration. While in the case of higher irradiation power, the rate of degradation is much more than the rate of desorption resulting in the overall decrease of the concentration of TCP as measured spectrophotometrically (Table 4). Thus it can be easily said that the destruction of TCP in the presence of TiO2 catalyst is a complex phenomena comprising of simultaneous degradation, desorption and adsorption processes and may be only physical as discussed on the basis of surface adsorption contribution. 3.4. Analysis of simultaneous degradation and desorption of TCP Table 4 gives the individual results of degradation and desorption of TCP in the solution based on the analysis of individual rates of each processes calculated as described earlier. It can be clearly seen from the table, that the percentage degradation with 30% amplitude is indeed high but at the same time, desorption rate is also
A.B. Pandit et al. / Ultrasonics Sonochemistry 8 (2001) 227±231
high. As the contribution of desorption process is also signi®cant, the overall concentration of free TCP which is being measured by estimating the rates of disappearance of TCP in solution is altered. It should be noted that the magnitudes of TCP adsorbed and hence affecting desorption/degradation processes are not significantly high when the catalyst loading of 0.01 g/100 ml of solution is used. This resulted into trends similar to that observed when the catalyst was not present though the actual degradation rates are likely to be marginally altered. Thus the adsorption and desorption rates must be considered for the estimation of correct degradation rates. Adsorption is not destruction but only a physical removal from liquid phase. 4. Conclusions The present work has enabled us to calculate the extent of adsorption±desorption of TCP on the photocatalyst TiO2 as well as decomposition of TCP under the in¯uence of ultrasound operating at varying amplitudes. The various conclusions, which can be made from the present work, are given below: 1. The degradation rates are higher in the presence of catalyst. 2. Increase in the intensity of irradiation by changing the power input to the system increases the rates of
231
degradation obtained. The eect observed is speci®c to the type of application and hence should not be generalized, as higher intensities are not necessarily better [6]. 3. The overall destruction process is a complex phenomena with simultaneous adsorption, desorption and the degradation by the ultrasound in the presence of the catalyst. 4. Rates of desorption and adsorption must be taken into account when the free amount of the material in the solution is determined with the help of spectrophotometer. References [1] N. Serpone, R. Terzian, H. Hidaka, E. Pelizzetti, J. Phys. Chem. 98 (1994) 2634. [2] C. Petrier, M. Lamy, A. Francony, A. Benahcene, B. David, V. Renaudin, N. Gondrexon, J. Phys. Chem. 98 (1994) 10514. [3] G. Thoma, J. Swoord, V. Popov, M. Som, Adv. Env. Res. 1 (1997) 178. [4] I. Hua, R.H. Hochemer, M.R. Homan, Env. Sci. Tech. 29 (1995) 2790. [5] A. Henglein, C. Kermann, Int. J. Radiat. Biol. 48 (1985) 541. [6] P.R. Gogate, A.B. Pandit, AIChE J. 46 (2000) 372. [7] I.Z. Shirgaonkar, A.B. Pandit, Ultrason. Sonochem. 5 (1998) 53. [8] T.J. Mason, J.P. Lorimer, Sonochemical Theory, Application and Uses of Ultrasound in Chemistry, Ellis Horwood, Chichester, 1998. [9] P.R. Gogate, M. Sivakumar, P. Senthilkumar, I.Z. Shirgaonkar, A.B. Pandit, AIChE J., accepted for publication (2001).
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Ultrasonic degradation of 2:4:6 trichlorophenol in presence of TiO2 catalyst Aniruddha B. Pandit *, Parag R. Gogate, Sukti Mujumdar Chemical Engineering Section, University Department of Chemical Technology (UDCT), Matunga Road, Matunga, Mumbai 400 019, India
Abstract The degradation of trichlorophenol has been studied at dierent intensities of irradiation using ultrasonic horn by changing the power input to the system. Eect of presence of catalyst TiO2 and concentration of catalyst on the degradation rates has also been investigated. The rates of degradation are found to be higher at higher intensities in the absence of catalyst but reverse trend is observed in the presence of catalyst. Adsorption and desorption characteristics of trichlorophenol on TiO2 catalyst have been examined. The catalyst enhances the rates of degradation but it also adsorbs some amount of TCP during the degradation process protecting it from ultrasonic degradation. Thus it is essential to consider the adsorption and desorption kinetics to get proper estimates of the degradation rates when the rates of degradation of TCP are calculated by analyzing the residual concentration of the compound in the liquid. The degradation of the pollutant seems to take place in the liquid only and that too only after desorption of the same from the solid particles. Solid particles seem to play a physical role in the overall degradation scheme, providing additional surface cavitation. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Waste water treatment; Ultrasonic irradiation; Adsorption and desorption characteristics; 2:4:6 trichlorophenol
1. Introduction The development of sonochemistry revealed a new and promising application for hazardous chemical destruction from industrial wastewater. Several complex compounds that have been successfully degraded, though on a laboratory scale at present, include 2-, 3and 4-chlorophenol [1], phenol [2], dichloromethane and o-dichlorobenzene [3], p-nitrophenol [4], etc. Sonication of aqueous solution provokes formation, growth and collapse of cavities resulting into local high pressures and temperature known as hot spots, which are responsible for the chemical degradation. Among the several modes of reactivity, pyrolytic decomposition theory is most widely accepted. According to this theory, during the formation of bubbles, resulting from expansion of cavities present in solution under sound ®eld, vapour from liquid medium or dissolved organic compound penetrate into these bubbles. These bubbles pulsate in the oscillating pressure ®eld till they reach a critical resonant size. At this stage the bubble can no
*
Corresponding author. Fax: +91-22-414-5614. E-mail address: [email protected] (A.B. Pandit).
longer eciently absorb energy from the sound ®eld to sustain itself. The bubbles then implode releazing large amount of energy, which is sucient to break strong chemical bonds in the compounds present in the bubbles as vapour. In aqueous solution, highly reactive OH & H radicals which are generated due to cavitation bombard the organic compound either inside or outer side of the bubbles according to the condition of the reaction and the half-life of the radicals resulting into oxidation reaction. On collapsing, the radicals present in the cavities are thrown into the bulk solution and are likely to be in contact with some dissolved organic molecules in the bulk resulting into oxidation reaction. Thus pyrolysis depends on the localization of solute (i.e. in the bulk, inside the cavities or in the interfacial layer) and therefore on its physico-chemical properties. Henglein and Kermann [5] reported that the hydrophobicity of solution is more responsible for the penetration of compound into the bubble as compared to liquid vapour pressure. Thus, hydrophilic organic compounds may get attacked by OH radical in the bulk solution or in the interfacial ®lm. In other words, hydrophobic and volatile compounds are destroyed very easily inside the bubble whereas non-volatile and hydrophilic compounds
1350-4177/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 5 0 - 4 1 7 7 ( 0 1 ) 0 0 0 8 1 - 5
228
A.B. Pandit et al. / Ultrasonics Sonochemistry 8 (2001) 227±231
grade TiO2 catalyst to study its eect. The amount of the catalyst used was varied in the range 0.01 g TiO2 /100 ml to 0.05 g TiO2 /100 ml of TCP solution. The adsorption isotherm of TiO2 catalyst was also studied by dispersing it in aqueous solution of TCP using Ace horn at 30% amplitude which gives maximum dispersion followed by the analysis of free TCP using the spectrophotometer. Desorption kinetics of TiO2 was also studied at 50°C which was the temperature reached due to heating of solution under uncontrolled sonication so that the equilibrium conditions could be studied. The time required to reach 50°C temperature from initial temperature of 30°C was 5 min for 30% amplitude of ultrasonic horn.
are more dicult to oxidize by ultrasound. Thus there is a need to improve the reactivity of these compounds which can be done partly by optimization of the geometry of sonoreactors and operational variables viz., frequency and intensity of ultrasound [6]. Accelerating the rate of the degradation of aqueous organic compounds by adding catalysts is another possibility to increase the eciency of degradation process. In the present work degradation of 2:4:6 trichlorophenol (TCP) has been studied applying ultrasound of dierent intensity and in the presence of a catalyst (TiO2 ) since TCP is well known as a hazardous toxic compound obtained from the euent of various chemical industries. The catalyst TiO2 has been purposefully chosen in the study to facilitate the use of UV irradiation in future as rate of degradation with combination of ultrasound and ultraviolet are higher than due to ultrasound or ultraviolet irradiation alone [7].
3. Results and discussion 3.1. Eect of presence of catalyst Table 1 shows the eect of presence of catalyst on the TCP destruction process. It is found that the degradation of TCP is higher under the in¯uence of the ultrasound in the presence of the catalyst TiO2 compared to that when catalyst was absent. This is due to the fact that the presence of heterogeneous catalyst increases the rate of formation of cavities by providing additional nuclei and also the number of cavities and radicals are increased. The cavitational threshold are also likely to be lowered in the presence of trapped vapour gas nuclei in the crevices of the particular catalyst [8]. Another possible explanation for the observed eect lies in the phenomena of interfacial cavitation at the catalyst surface. Due to the high pressure and temperature pulses generating as a result of cavitation phenomena, fragmentation of catalyst occurs which again provides more number of nuclei thereby increasing the number of cavities and consequently the degradation of TCP present in the solution. It can be also seen from Table 1, that the extent of degradation is also dependent on the concentration of the catalyst present in the reaction mixture. This can be attributed to the fact that the number of nuclei increased as a result of heterogeneous surface and the extent of interfacial cavitation also increases with the quantity of the catalyst. Moreover the rate of degradation of the
2. Experimental The solution (100 ml of 100 ppm concentration of TCP) was sonicated by ultrasonic horn (Ace Glass Inc. USA, operating frequency 22:7 kHz and power rating 600 W) in a pulse mode (5 s on and 1 s o) at room temperature. A schematic representation of the experimental setup has been depicted in Fig. 1. The effect of intensity was studied using dierent amplitude (10%, 20% and 30%) of ultrasonic horn. By changing the amplitude of irradiation, the power input to the system is changed keeping the area of irradiation constant. The residual concentration of TCP was determined using UV/VIS spectrophotometer at a wavelength of 293 nm at intervals of 30 min during a total irradiation time of 2 h. Experiments were repeated in presence of anatase
Fig. 1. Schematic representation of the experimental set-up.
Table 1
Eect of presence of catalyst on the destruction of TCP Time (min)
50 150
Percentage decrease in concentration of TCP Without catalyst
In presence of the catalyst Concentration 0:01 g/100 ml
Concentration 0:05 g/100 ml
11.07 32.12
24.23 66.6
40.37 82.12
Experimental conditions: amplitude 10%, power input to system 60 W.
A.B. Pandit et al. / Ultrasonics Sonochemistry 8 (2001) 227±231
organic compound is proportional to the amount of OH and H radicals produced and present in the reaction mixture and the presence of TiO2 catalyst enhances the dissociation reaction of H2 O thereby increasing the number of free radicals generated. 3.2. Eect of intensity of irradiation Intensity of irradiation can be de®ned as the ratio of the power input to the system to the area of transmitting surface. Intensity of irradiation can be changed in two ways; ®rst by changing the power input or second by changing the irradiating surface area. An increase in the irradiating surface area results into more uniform dissipation of energy and that too over a wider area resulting into more eective of cavitation [6]. Thus the cavitational yield is inversely proportional to the intensity of irradiation in this case. Gogate and Pandit [6] have shown using numerical simulations of the cavity dynamics that, the collapse pressure generated due to the collapse of individual cavity decreases with an increase in the intensity of irradiation and recommended the use of lower intensity i.e. just above a certain optimum required for the inception of cavitation de®ned as threshold intensity. In the present case, the intensity of irradiation has been increased by increasing the power input to the system. The eect of intensity of irradiation is distinctly dierent for the two cases viz., one in absence of catalyst and other in presence of catalyst, which is discussed in details in the following sections. 3.2.1. In absence of catalyst TiO2 In the degradation of TCP solution, it was observed that in the absence of catalyst, the extent of degradation increases with an increase in the power of the sonic horn. This is due to the increased cavitational activity at higher levels of power input. Due to an increase in the power input to the solution, higher number of cavities are generated resulting into higher magnitudes of total
229
pressure pulses (pressure pulse due to collapse of single cavity number of cavities) though the pressure pulse due to collapse of single cavity decreases [6]. Thus the overall cavitational yield increases as is observed in the present work. However it should be noted that the increase in the number of collapsing (eective) cavities with an increase in power input is only upto a particular intensity after which the coalescence and buoyancy renders the additional number ineective. Gogate et al. [9] have clearly shown this fact with experimentation on the decomposition of aqueous solution of potassium iodide. 3.2.2. In the presence of catalyst TiO2 The eect of intensity on the destruction rates in presence of catalyst was studied at two catalyst concentrations viz., 0.01 g/100 ml of solution and 0.05 g/100 ml. The trends in variation of degradation rates for the concentration of 0.01 g/100 ml of solution (Table 2, Case II) were similar to those observed in the absence of catalyst. However, during sonication of TCP with 0.05% TiO2 , the degradation of TCP was less at higher power input, which was quite contrary to the expectation (Table 2, Case III). The observed variation can be explained on the basis of the adsorption and desorption characteristics of TCP on TiO2 catalyst. At higher power input, though the degradation rate in the solution increases, the rate of release of already adsorbed TCP also increases thereby giving lower overall degradation rate based on the free concentration of the TCP in the liquid. The detailed explanation of this fact has been made in the later section on the basis of adsorption and desorption characteristics of TCP on TiO2 . 3.3. Adsorption and desorption characteristics The extent of adsorption of TCP by TiO2 was found to increase with time of contact of TiO2 and TCP solution (Table 3, Part I). During irradiation, the particles of TiO2 experience fragmentation due to shock waves
Table 2 Eect of intensity of irradiation on the degradation rates Time (min)
Percentage decrease in concentration of TCP 30% amp Power 180 W
20% amp Power 120 W
10% amp Power 60 W
42.12 74.05
24.49 48.96
11.07 32.12
Case II: In the presence of catalyst (0.01 g/100 ml of solution) 50 83.7 150 95.2
75.9 89.2
24.23 66.60
Case III: In the presence of catalyst (0.05 g/100 ml of solution) 50 19.32 150 76.85
34.23 80.37
40.37 82.12
Case I: Without presence of catalyst 50 150
230
A.B. Pandit et al. / Ultrasonics Sonochemistry 8 (2001) 227±231
Table 3 Adsorption and desorption characteristics TCP on the catalyst TiO2 Time (h)
Concentration of free TCP (ppm)
ppm adsorbed/gm of catalyst
Part I: Adsorption isotherm of TCP on TiO2 (concentration of 0.05 g/100 ml of solution) 0 78.67 1 26.65 2 22 3 19.92 Time (h)
Concentration of TCP (ppm) Temperature 65°C
Temperature 50°C
Part II: Study of desorption of TCP at elevated temperatures 3h 18.75 3 h 30 min 39.19 4h 42.35 Time for irradiation (min)
± 1040.4 1133.4 1175.5
18.31 24.46 25.68
Concentration of TCP (ppm) With 30% amp power 180 W
With 10% amp power 60 W
Part III: Eect of ultrasound on equilibrated TCP (containing TiO2 ) solution 0 21.12 25 44.02 50 36.30 75 28.5
generated by imploding cavities in the bulk of the solution. The shock waves damage, fragment and deform the solid surface leading to enhancement of surface area and improvement of mass transport due to turbulent mixing and acoustic streaming from the bulk to the catalyst surface. Desorption characteristics were studied at 65°C and 50°C which are the temperatures reached due to heating caused during uncontrolled irradiation with 30% and 10% amplitude respectively in 15 min irradiation time (Table 3, Part II). It can be easily seen from the table, that there is an increase in the concentration of free TCP in the solution after irradiation of the equilibrated sample which con®rms the fact that desorption is also taking place. When the equilibrated sample of TCP and TiO2 was irradiated with ultrasound at dierent amplitudes (Table 3, Part III), it was observed that upto certain irradiation time, the concentration of TCP increases in the solution because of desorption of the TCP from TiO2 present in the solution under the in¯uence of ultrasound. Afterwards, the concentration of TCP increases gradually with time for lower irradiation power (10% amplitude) while, for higher power input (30% amplitude), it decreases with time as expected and observed in the absence of catalyst. This can be explained considering the rates of simultaneous degradation and desorption during sonication. The rate of degradation is less as compared to the rate of desorption from the catalyst in the case of lower irradiation power which ensures that there is more supply of TCP by desorption as compared to removal by degradation thereby increasing the overall
19.19 41.30 52.44 53.49
Table 4 Analysis of individual contribution of degradation and desorption Time (min)
25 50 75
With 30% amp Power 180 W
With 10% amp Power 60 W
% Degradation
% Desorption
% Degradation
% Desorption
34.14 42.12 50.11
12.97 12.76 2.6
5.80 11.07 16.33
35.18 45.07 45.91
concentration. While in the case of higher irradiation power, the rate of degradation is much more than the rate of desorption resulting in the overall decrease of the concentration of TCP as measured spectrophotometrically (Table 4). Thus it can be easily said that the destruction of TCP in the presence of TiO2 catalyst is a complex phenomena comprising of simultaneous degradation, desorption and adsorption processes and may be only physical as discussed on the basis of surface adsorption contribution. 3.4. Analysis of simultaneous degradation and desorption of TCP Table 4 gives the individual results of degradation and desorption of TCP in the solution based on the analysis of individual rates of each processes calculated as described earlier. It can be clearly seen from the table, that the percentage degradation with 30% amplitude is indeed high but at the same time, desorption rate is also
A.B. Pandit et al. / Ultrasonics Sonochemistry 8 (2001) 227±231
high. As the contribution of desorption process is also signi®cant, the overall concentration of free TCP which is being measured by estimating the rates of disappearance of TCP in solution is altered. It should be noted that the magnitudes of TCP adsorbed and hence affecting desorption/degradation processes are not significantly high when the catalyst loading of 0.01 g/100 ml of solution is used. This resulted into trends similar to that observed when the catalyst was not present though the actual degradation rates are likely to be marginally altered. Thus the adsorption and desorption rates must be considered for the estimation of correct degradation rates. Adsorption is not destruction but only a physical removal from liquid phase. 4. Conclusions The present work has enabled us to calculate the extent of adsorption±desorption of TCP on the photocatalyst TiO2 as well as decomposition of TCP under the in¯uence of ultrasound operating at varying amplitudes. The various conclusions, which can be made from the present work, are given below: 1. The degradation rates are higher in the presence of catalyst. 2. Increase in the intensity of irradiation by changing the power input to the system increases the rates of
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degradation obtained. The eect observed is speci®c to the type of application and hence should not be generalized, as higher intensities are not necessarily better [6]. 3. The overall destruction process is a complex phenomena with simultaneous adsorption, desorption and the degradation by the ultrasound in the presence of the catalyst. 4. Rates of desorption and adsorption must be taken into account when the free amount of the material in the solution is determined with the help of spectrophotometer. References [1] N. Serpone, R. Terzian, H. Hidaka, E. Pelizzetti, J. Phys. Chem. 98 (1994) 2634. [2] C. Petrier, M. Lamy, A. Francony, A. Benahcene, B. David, V. Renaudin, N. Gondrexon, J. Phys. Chem. 98 (1994) 10514. [3] G. Thoma, J. Swoord, V. Popov, M. Som, Adv. Env. Res. 1 (1997) 178. [4] I. Hua, R.H. Hochemer, M.R. Homan, Env. Sci. Tech. 29 (1995) 2790. [5] A. Henglein, C. Kermann, Int. J. Radiat. Biol. 48 (1985) 541. [6] P.R. Gogate, A.B. Pandit, AIChE J. 46 (2000) 372. [7] I.Z. Shirgaonkar, A.B. Pandit, Ultrason. Sonochem. 5 (1998) 53. [8] T.J. Mason, J.P. Lorimer, Sonochemical Theory, Application and Uses of Ultrasound in Chemistry, Ellis Horwood, Chichester, 1998. [9] P.R. Gogate, M. Sivakumar, P. Senthilkumar, I.Z. Shirgaonkar, A.B. Pandit, AIChE J., accepted for publication (2001).